What is Mutating Table?
here is the answer,
Mutating Table is a table that is currently being modified by an Insert, Update or Delete statement. Constraining Table is a table that a triggering statement might need to read either directly for a SQL statement or indirectly for a declarative Referential Integrity constraints. Pseudo Columns behaves like a column in a table but are not actually stored in the table.
For example, if your trigger contains a select statement or an update statement referencing the table it is triggering off of you will receive the error.